This invention relates in general to Electronic Brake Control Systems and in particular to a four channel Electronic Brake Control System that utilizes three wheel speed sensors.
An Electronic Brake Control System (EBCS) is often included as standard equipment on new vehicles. One of the most common such systems is an Anti-lock Brake System (ABS). When actuated, the ABS is operative to control the operation of some or all of the vehicle wheel brakes to prevent wheel lock-up during a brake application cycle. Another EBCS can include Traction Control (TC) which is operative to selectively brake driven wheels that are slipping during vehicle start-ups on low friction surfaces, such as ice or snow. Because of similarity of hardware, an EBCS with TC capability usually includes an anti-lock braking function. A third EBCS can include Vehicle Stability Control (VSC) which is responsive during turning and other vehicle maneuvers to selectively actuate individual wheel brakes to enhance the directional control of the vehicle. Again, due to the similarity of the hardware involved, an EBCS with VSC capability also usually includes both anti-lock braking and TC functions.
Referring now to the drawings, there is illustrated in FIG. 1 a schematic diagram of a typical hydraulic brake system 10 for a four wheel drive vehicle that includes a prior art EBCS capability. The brake system 10 is intended to be exemplary and it will be appreciated that there are other brake control systems having more or less components. While the brake system 10 shown is for a four wheel drive vehicle, a same system also could be used on a two wheel drive vehicle, with either front or rear wheel drive. The system 10 includes a brake pedal 12 that is mechanically connected to a brake light switch 12 and a dual reservoir master cylinder 14. A first reservoir of the master cylinder 14 supplies hydraulic fluid a front wheel brake circuit while a second reservoir provides hydraulic brake fluid to a rear wheel brake circuit.
The first reservoir in the master cylinder 14 is connected to an EBCS control valve 16 by a first hydraulic line 18 while the second reservoir is connected to the control valve 16 by a second hydraulic line 20. The EBCS control valve 16 includes plurality of normally open and normally closed solenoid valves (not shown) and a separate source of pressurized hydraulic fluid, such as a motor driven pump (not shown). The pump is typically included within the body of the control valve 16 while the pump motor is mounted upon the exterior thereof.
The control valve 16 is connected by a first pair of hydraulic brake lines 22 and 24 to right and left front vehicle wheels 28 and 30, respectively. For the vehicle shown in FIG. 1, the front wheels 28 and 30 are both driven and steerable. However, as indicated above, EBCS can also be applied to vehicles having non-driven front wheels. Similarly, a second pair of hydraulic brake lines 26 and 27 connect the control valve 16 to right and left rear vehicle wheels 32 and 33, respectively.
The control valve 16 can include one of several configurations. For example, the control valve 16 may include a normally open solenoid valve (not shown) between each of the brake circuits and the corresponding master cylinder reservoir. Upon actuation, the valve closes to isolate the brake circuit from the master cylinder 14. Accordingly, the normally open valve is typically referred to as an isolation valve. For optimal control of the speed of each of the vehicle wheels, each of the wheel brakes may be provided an associated isolation valve. The control valve also typically includes a first normally closed valve (not shown) for each wheel brake that connects the wheel brake cylinder with a brake fluid reservoir (not shown). Upon actuation, the first normally closed valve is opened to bleed hydraulic fluid from the wheel brake cylinder and thereby reduce the pressure applied to the wheel brake. Accordingly, the first normally closed valve is usually referred to as a dump valve. The control valve also usually includes a second normally closed valve (not shown) for each wheel brake that connects the wheel brake cylinder with a outlet of the pump. Upon actuation, the first normally closed valve is opened to supply pressurize hydraulic fluid from the pump to the wheel brake cylinder and thereby raise the pressure applied to the wheel brake. Accordingly, the second normally closed valve is usually referred to as an apply valve. Usually, the reservoir connected to the dump valves is connected to the pump inlet to supply hydraulic brake fluid to the motor driven pump.
Alternately, the functions of the apply and isolation valves may be combined into a single normally open valve that also is referred to as an apply valve. This configuration simplifies the control valve 16 by eliminating dedicated isolation valves for the front and rear brake circuits. In such as case, the master cylinder 14 is not separated from the pump while the EBCS is activated. Because both of the control valve configurations described above allow independent control of each of the four vehicle wheel brakes, there are four control channels and the EBCS if usually referred to as a four channel system.
The speed of the front wheels 28 and 30 are monitored by a first pair of associated wheel speed sensors, 34 and 35, respectively. Similarly, the speed of the rear wheels 32 and 33 are monitored by a second pair of associated wheel speed sensors 36 and 37, respectively. The wheel speed sensors 34, 35, 36 and 37 are electrically connected to an EBCS Electronic Control Unit (ECU) 38. The ECU 38 is also electrically connected to the bake light switch 38. Closing the brake switch 13 provides a signal to the ECU 38 that the vehicle brakes have been activated in support of ABS functions. The ECU 38 also is electrically connected to the pump motor and the actuation coils of the solenoid valves included with the control valve 16. The ECU 38 can further be connected to vehicle directional sensors, such as yaw and lateral acceleration sensors and steering angle sensors (not shown) and other sensors to provide status of vehicle components, such as a throttle position sensor (not shown). Such sensors are used to support TC and VSC functions. The ECU 38 includes a microprocessor with a memory that stores an EBCS control algorithm.
During vehicle operation, the microprocessor in the ECU 38 continuously receives speed signals from the wheel speed sensors 34, 35, 36 and 37 and, when equipped vehicle directional and other status sensors. Upon detecting the potential of problem that could be remedied by the actuation of the EBCS, the control algorithm cyclically actuates the solenoid valves in the control valve 16 to selectively apply the wheel brakes and correct the problem. For example, excessive wheel slip during a braking cycle is an indication of potential wheel lock-up. Upon detecting such a potential problem, the control algorithm would cause the ECU 38 to implement an ABS braking cycle. Similarly, for an EBCS with TC capability, excessive wheel slip during a vehicle start-up is an indication of loss of traction. Upon detecting such a potential problem, the control algorithm, would cause the ECU 38 to implement a TC cycle. For an EBCS with VSC capability, detection of an indication of a potential loss of directional control, the control algorithm would cause the ECU 38 to implement a VSC cycle to correct the vehicle's direction.